The present invention relates to an enhanced peak detector circuit, and more particularly to a simple and inexpensive peak detector circuit for use in electronic identification devices that effectively functions as an amplitude demodulator circuit that extracts an encoded transponder return signal from the carrier signal.
Identification devices and systems are known in the art. See, e.g., U.S. Pat. Nos. 3,859,624; 3,878,528; 4,095,214; 4,129,855; 4,354,099; 4,475,481; 4,549,264; and 4,739,328. Typically, such devices and systems are used for (1) access control, (2) animal feeding and animal health, (3) inventory control, (4) process control, and/or (5) theft/security applications.
Many of the aforementioned identification systems include: (1) a transponder that transmits a unique identification signal; and (2) an identifier that interrogates the transponder, then receives and decodes the identification signal. The transponders are typically small, inexpensive, devices that perform the dedicated function of generating and transmitting their unique identification signal. The transponders are placed on, or carried by, objects, animals, or persons that are to be identified. The identifiers are placed at or near a location where an identification is to be made. Hence, when an object, animal, or person carrying a transponder comes near an identifier, the identifier is able to interrogate the transponder and trigger the generation of the identification signal, receive the identification signal, and decode the identification signal so as to identify the particular object, animal, or person to which the transponder has been attached. In response to making such a valid identification, the identifier may then generate appropriate control signals that carry out a desired function, e.g., the unlocking of a door, the dispensing of a prescribed amount of food, the turning on of a video camera or audio recorder, the time logging of the identification signal, the enabling of a piece of machinery, and the like.
Some identification systems may utilize a transmitter to generate the identification signal, as opposed to a transponder. In such instances, the transmitter periodically generates and transmits the identification signal, e.g., once every 10 seconds, regardless of whether the transmitter is near an identifier. However, the identification signal is of limited range, so the identification signal is only received by the identifier when the transmitter is in close proximity thereto.
The present invention relates primarily to the identifier device or apparatus. Equipment typically used to perform the function of the identifier apparatus in an electronic identification system is generally one of two types. A first type is a "portable" identifier used to read transponders on animals, packages, or people. Such portable identifier typically is about the size of a clothing iron, and may weigh several pounds. Some older portable identifiers or readers are much larger, and are so bulky that they must be placed on a strap and hung over the shoulder of a user. But even when only the size of a clothing iron, the carrying and positioning of such identifier apparatus can become tiring and burdensome. Thus, there is a need in the art for a smaller, more compact, and light weight identifier device.
The second type of identifier device used in the art is a "stationary" identifier, typically permanently housed in a box mounted on a wall or door, and used to read a transponder-type security badge. In most cases, in order to keep the size of the box small and unobtrusive, an antenna and a minimum amount of circuitry is housed in the box, while the remaining circuitry is housed in another (usually hidden) box mounted in the ceiling or elsewhere. The separating of the circuitry between two or more boxes in this manner results in increased expense, maintenance and circuit complexity.
A more recent electronic identification apparatus is disclosed in U.S. Pat. No. 5,347,263 issued on Sept. 13, 1994 to inventor-applicants Gary Carroll and J. Donald Pauley. This simple, low-cost electronic identification system provides a reader or identifier that includes an antenna coil, a single chip microcontroller with a low cost ceramic resonator or crystal connected directly to the antenna coil, a peak detection circuit connected to the antenna coil, a band pass amplifier connected to the peak detector or other demodulator circuit, and an I/O interface circuit connected to the single chip microcontroller.
In this related art system, the single chip microcontroller is adapted to generate a power output signal or interrogation signal that is applied to the antenna coil. The microcontroller is also adapted to decode any identification signal that is received by the system through the antenna coil, demodulator circuit, and band pass amplifier so as to identify the particular object, animal, or person to which the transponder has been attached.
While the system described in the '263 patent has advantageously provided an electronic identification apparatus having very few parts and can thus be made relatively inexpensively and compactly, it requires a higher speed, larger memory microprocessor to perform all of the required decoding and signal generating functions within the short response time demanded by many electronic identification applications. Furthermore, the '263 patented device requires a crystal or other resonating device to clock the microprocessor and synchronize the system.
Also seen in the '263 patent, the decoding function of electronic identifiers typically involves some sort of amplitude demodulation. In the related art systems, it is necessary to extract the identification signal returned from the transponder from the power signal. This extraction may be accomplished through amplitude demodulation wherein the identifier unit often includes a peak detector circuit and a multiplicity of filters in order to accurately extract the modulated signal from the power carrier. Peak detector circuits have been known for quite some time and used extensively in radio receivers to recover amplitude variations from an incoming carrier signal. Most conventional peak detector circuits are diode based peak detector circuits that comprise a specific arrangement of diodes, capacitors, resistors, comparators, re-set switches, etc., which hold the positive peak or negative peak or both of the input voltage on a peak detecting capacitor.
Alternatively, electronic identification systems are built which utilize elaborate inductor-capacitor filter designs without a peak detector circuit to separate the desired signal (of one frequency) from the wide spectrum of frequencies present in the carrier signal. Still other designs of electronic identification systems use conventional peak detectors along with less elaborate filters such as a multi-stage band pass amplifier.
What is thus needed is a small identifier device that can perform all the decoding and signal generating functions needed by electronic identification devices using inexpensive circuitry that does not require the use of a costly crystal or ceramic resonator, and that can also utilize a relatively low cost microprocessor.
There is also a need for a simple and inexpensive means for providing the interrogation signal generation function and to clock the microprocessor in electronic identification devices with relatively low distortion and with a high ratio of energy stored to energy dissipated and radiated by the antenna coil so that the electronic identification device is less likely to be affected by lossy surroundings.
Finally, there is also a need for a simple and inexpensive means for extracting or recovering identification signals that are modulated at very low modulation levels and/or at relatively low frequency ratios, with respect to the interrogation or carrier signal frequency, in order to minimize the need for elaborate filtering techniques.